Rosemount, Minnesota (CNN) -- Andrew Wilfahrt changed his gait in the weeks before going off to basic training. He walked more upright. He bulked up with weights. He spoke with a deep Robocop voice. He acted "manly."

Through the eyes of his parents, Jeff and Lori, it was all a bit strange.

This was the boy who told them he was gay at 16 after being confronted with exorbitant bills from Internet chat rooms. Who lobbied for gay rights in his high school and escaped the fists of football players when hockey players came to his rescue. Who had the courage to wear pink and green even after his car was spray-painted with "Go Home Fag!"

All his parents ever wanted was for Andrew to be Andrew.

At 29, he sat his mom and dad down at the kitchen table and told them his life was missing camaraderie, brotherhood. "I'm joining the Army," he said.

The news surprised them. Why would Andrew enter the military, where he'd be forced to deny a part of who he is?

He was a lover of classical music, a composer, a peace activist, a math genius. He studied palindromes, maps, patterns, the U.S. Constitution, quantum physics.

A soldier?

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It had never really crossed the minds of his left-leaning parents. Yet, just as they'd done with all three of their children, they supported him. It wasn't easy. It became dreadfully painful.

When their son wound up in Afghanistan in July 2010, Jeff awoke early each day to Google "Kandahar." He tracked every soldier killed in the far-off land.

Then, on February 27, 2011, at the same oak table where Andrew said he was joining up, the Wilfahrts learned their oldest child was gone.

"I want to talk directly to somebody in his platoon!" Jeff told the officer and chaplain seated across from him. He wanted to know for sure that this wasn't a behind-the-shed killing of the gay guy.

Cpl. Andrew Charles Wilfahrt, 31, is believed to be the first gay U.S. soldier to die in battle since President Obama signed the repeal of "don't ask, don't tell," the policy forcing gays in the military to hide that part of their lives or risk being kicked out.

He was also among the smartest in the half-million force, scoring a perfect score on his aptitude test, a feat the Army says is rare.

Andrew was so well-liked his comrades named a combat outpost for the soldier with the infectious smile. COP Wilfahrt sits 6 kilometers from Kandahar. To his buddies, it is not named for a gay soldier, but for one who fought with valor.

"Mom, everyone knows. Nobody cares," he told his mother in their final conversation, a phone call from Afghanistan on Thanksgiving.

In a biography he left on his laptop, Andrew described himself as someone who "espoused casual solipsism, the idea that ultimately one can know only oneself and nothing more.

"Although close to my parents and siblings, I generally prefer solitude and introspection, and have but few close associates," he wrote.

"I have maintained 'bachelor status' with the strictest of discipline, and a discipline I secretly wish would be compromised by a charming beauty."

Andrew never denied his sexuality. But like so many, he struggled with what it means to be gay in America. Yet it was only one part of him. He was so much more. In the note on his laptop, he never used the words gay or homosexual to define himself. His younger sister, Martha, says it's the least interesting thing about him.

But with his death, his parents have taken up the cause of gay rights. Andrew fought for his nation in a foreign land. His parents' war is being waged in their home state of Minnesota. To them, it's about defending the Constitution -- protecting the rights of all citizens.

Gay in the land of Pawlenty, Bachmann

The red Toyota Corolla eases through the streets of downtown Minneapolis. The Wilfahrts are entering a part of their son's world that was distant to them. They're headed from their home in suburban Rosemount to the Twin Cities Pride Parade, an annual event their son loved.

"It's new for us," Lori says .

They ride in solemn silence. Harry Nilsson sings from the speakers:

"Remember, life is just a memory Remember, close your eyes and you can see Remember, think of all that life can be Remember, dream, Love is only in a dream ..."

His mother puts her hands to her face and cries. Her son's dream was to fall in love and find a job that allowed time to compose music.

"Are you OK, honey?" Jeff asks his wife.

The two have been married for 33 years. Lori works as a project manager for 3M. Jeff had a career there as well, but has been unemployed since the beginning of the year.

Lori and Jeff Wilfahrt attend their first gay pride parade. They hold a flag signed by their son's comrades.

The Wilfahrts have the milquetoast looks of middle-age Midwesterners: gray hair, rimmed glasses, apple-pie ordinary. Yet make no mistake: These lifelong Minnesotans might be the most powerful force to join the same-sex marriage movement.

In a state that has produced GOP presidential hopefuls Michele Bachmann and Tim Pawlenty -- who have made careers fighting gay marriage -- these parents of an American hero present a major challenge to the establishment.

They'll take their battle to the Supreme Court, if that's what it takes. To the Wilfahrts, denying gays the right to marry is discrimination against a group to which their son belonged.

Jeff has asked Lady Gaga to come to Minnesota to dance a same-sex marriage polka. He skipped a recent White House tea with the first lady held for families of service members. He wanted to send a message to the Obama administration: My son gave his life for his country, yet didn't have full rights back home.

On a recent spring day, the couple stood outside the Capitol while lawmakers inside prepared to debate marriage. The legislators voted, largely along party lines, to put a constitutional amendment on the ballot for November 2012 to define marriage as solely between a man and woman.

Jeff had never spoken much publicly before eulogizing his son. He began by telling the crowd, "If I hold my finger up, I'm gonna be crying. When you see that, I need to pause."

A few minutes later, his finger dangled in the breeze. His voice cracked. "I challenge the one-man, one-woman champions to define manliness or womanhood. Will you as a human being, as an American, as a Minnesotan, be asked to open your trousers or to have your skirt lifted when applying for a license to marry?

" ... I hope my son didn't die for human beings, for Americans, for Minnesotans who would deny him civil rights."

On this day, in the grandstands of the pride parade, the Wilfahrts will celebrate their son's identity as both a gay man and a soldier. It's the type of event that would stun Bachmann and Pawlenty: More than 100,000 gays, lesbians, bisexuals, transgenders and straights gathered in their home state, celebrating life and obeying the law. A Minneapolis police car led the parade, two officers waving to the jubilant crowd.

The night before, Jeff, 58, and Lori, 56, wondered if they were doing the right thing by coming. Their son was so private, would he want his mom and dad to speak out?

Within minutes today, they get their answer. "Thank you for you and your son's service," a man says, offering a hug to Lori. Tears well in the parents' eyes.

Another stranger, Laurie Kermes, holds Lori's hand. "Your son did a lot. He's not going to be lost in vain."

Soon, a float goes by carrying two poster-sized photographs of Andrew in Army camo. "That's our boy!" Jeff says.

He and Lori embrace. Their heads tilt toward the ground, two exhausted parents missing their son.

'I am here to serve'

Andrew met with a gay former Marine in Minneapolis bars and coffee shops in the months before signing up. He wanted to know the pros and cons of being gay in the military.

He'd been volunteering at food shelters, animal shelters, an AIDS hospice, voter-registration drives and other non-profit initiatives. At 29, he was living with his parents and looking for more out of life.

The Marine says Andrew told him he wanted to serve so a soldier with a wife and children wouldn't have to go fight.

"He wasn't making a statement" about being gay. "He was doing it for everybody else," says Dan, who asked that his last name not be used. "He will forever be my hero because he joined for the right reasons. He was a silent part of the gay community, but it's just unspeakable how big of an impact he's had now."

His name and face have been front and center in the state's debate on gay marriage.

Cpl. Andrew Wilfahrt was just two days from leave from the Afghanistan theater when he was killed.

Republican Rep. John Kriesel, who lost his legs while serving in Iraq, sent Andrew's photo around the floor during debate in the Minnesota House.

A few years ago, he said, he would have defined marriage as solely between heterosexuals. But his military service changed that.

"This amendment doesn't represent what I went to fight for," he told lawmakers.

"I cannot look at this family and look at this picture and say, 'You know what, Corporal, you were good enough to fight for your country and give your life, but you were not good enough to marry the person you love.' I can't do that."

Andrew didn't have a significant other. If he had, the partner wouldn't have been allowed to escort his body home from Dover Air Force Base, nor would he have received Andrew's $100,000 death benefit.

Andrew arrived at Fort Leonard Wood in Missouri in February 2009. The man with the muscle-builder chest and six-pack abs drew immediate attention when quizzed by drill sergeants. He spoke in a Robocop voice. He asked question after question.

Watching him, Kevin Gill wondered: Who is this guy?

"After we became really close, he told me that was his 'tough man voice' and that he used it to show his real 'manhood,'" Sgt. Gill told CNN in a series of e-mails from Afghanistan.

Andrew earned the nickname Slovak for his macho speak and exaggerated, arrow-straight gait. Andrew was like that, a ham who figured out a way to fit in. When he laughed, he threw his head back, closed his eyes and let out a sound that made everyone else chuckle.

In combat, he rode with two other soldiers. One was African-American, the other from Hawaii. They were known as "Team Minority."

Intelligent didn't even begin to describe him. Everybody felt smarter just being around him. Shortly after Andrew arrived on post in Hawaii, a commander saw his perfect aptitude score and grilled him: What was somebody with such smarts doing as a grunt?

The Wilfahrts say they will fight for gay rights all the way to the Supreme Court.

"Is this some kind of joke, Wilfahrt?"

"No, sir," he said. "I am here to serve!"

Gill once asked him about World War I. Over the next week, for four hours a day, Andrew recounted the history of the first World War and all the other U.S. conflicts up through Vietnam.

Andrew felt a connection to World War I: His great-grandfather, Charles Wilfahrt, entered battle in the European theater on September 26, 1918. Ninety-two years to the day, Andrew entered Operation Dragon Strike in Afghanistan as a member of the 552nd Military Police Company.

The coincidental timing wasn't lost on him. He always found meaning in numbers.

The numbers with meaning for him and Gill were their ages. They bonded at boot camp, where they were the "old" guys. Andrew was 29 at the time. Gill was 39.

The two were like brothers. Their only difference before going to war: Gill headed to the straight bars during off-time at Schofield Barracks in Hawaii. Andrew hit the gay bars. Gill says Andrew always thought military investigators were following him.

None of his comrades cared about his sexuality. And, guys being guys, they cracked gay jokes around Andrew. His response: to laugh with them.

He said it was funny that he talked more about his sexuality with his band of brothers than he ever had with gay friends.

Gill paid close attention and made sure the jokes never got out of hand. One of his own brothers is gay and moved to Switzerland in the mid-1990s. The two haven't seen each other in 16 years, even though he accepts his brother for who he is. "That's the tough thing about it."

Gill says it helped to talk to Andrew. No topic was taboo. They shared everything: about family, life, the war. Andrew told him how hard it can be to be gay in America.

One day last fall, the two were doing guard duty at a tower in a Kandahar police station when Gill's understanding of what Andrew meant deepened. Andrew was reading a copy of Time magazine. In it was an article about gay teens who committed suicide after being bullied.

Andrew began to weep.

"This was more than just a tearful cry. This was all his emotion from the past just coming out all at once in front of his fellow soldier."

Andrew's parents say he struggled with suicidal tendencies in his early 20s. But every time, the thought of the four people he loved most -- his mom and dad, sister Martha and brother Peter -- stopped him.

In Afghanistan, Andrew confided in Gill.

"I just trusted him and was proud to be serving next to him right there on the battlefield."

'A damn good soldier'

It was Sunday, February 27.

Members of the 1st Squad, 3rd Platoon were on foot patrol in a region west of Kandahar, accompanied by members of the Afghan National Police.

There were 11 of them, and they were familiar with the area. Andrew was ninth in line as they crossed a bridge toward a police checkpoint. Children scattered.

A 122-mm mortar round lay hidden along the route.

Kevin Gill wrote this note on an American flag signed by Andrew Wilfahrt's comrades.

At 11:48 a.m., the massive bomb detonated beneath Andrew. Three other explosives, daisy-chained together, failed to go off. Gill was 20 meters ahead of his battle buddy. He'd have been killed, too, if the other bombs had exploded. He rushed toward Andrew. A medic joined. They were at his side within seconds.

It felt like a terrible training session. But it was all too real. Andrew's legs were blown off, as was his left hand. He'd suffered severe wounds to his head.

Andrew was the 66th Minnesotan to be killed while serving in the wars in Afghanistan or Iraq. Some 7,000 miles away, in the Wilfahrt home in Rosemount, the world shattered.

Their firstborn, the baby who had taken 12 hours of labor to deliver.

The boy, who at 6 asked his father: "Do you think there is a different kind of gravity at the edge of the universe?"

The man who told them he loved his band of brothers so much he hoped to become an Army lifer.

He was gone.

Four months after their son's death, Jeff and Lori sit at the kitchen table, the place where Lori says "a lot has gone down." They both say the Army's been good to them. They don't feel anger, except as Jeff puts it, for "those f--kers at the Capitol" who voted against same-sex marriage.

The document is inside a manila envelope with these words on the outside:

"WARNING: The information in the enclosed report is graphically described for complete accuracy in the physical details of the remains of Andrew C. Wilfahrt.

"It is STRONGLY RECOMMENDED that you read this in the presence of people that can provide you with emotional support during this time, such as your minister, a family friend, or a counselor."

Jeff and Lori read the detailed eight-page report alone, on their own time.

The anti-war activists whose boy once attended protests alongside them never thought they'd find themselves here, boasting about a soldier son. But they swell with pride, patriotic pride and gay pride.

"He died for the soldier to the left and right of him," he says. "And he was a damn good soldier."

Shortly after Andrew's death, Jeff wrote a letter to his son's comrades. "A gay child will take you to places in your heart you did not know existed," he said. "Regardless of orientation, I beseech all of you who are parenting now, or do so in the future, to give them all the love you can muster. At times it feels like you are bailing the ocean, but do not stop loving your children."

Ashes at kitchen table

The soldiers of the 552nd are preparing to return home after a year in theater. They will leave behind Combat Outpost Wilfahrt.

"We will never forget him and are honored to have served with such an outstanding person," platoon leader 1st Lt. Brandon LaMar said in a letter informing the family of the naming of the outpost.

That letter arrived on May 7, what would have been Andrew's 32nd birthday. Included in the package were memorial bracelets. The Wilfahrts wear theirs every day.

Their home has become a shrine.

Some of Andrew's ashes rest in a brown container near the family table. His photograph is taped to the outside. Nearby are two teddy bears, one tattered from his youth, one given to the family in his memory.

Jeff's greatest regret is not hugging his son when he first told him he was gay. "This is how it is for an old fool of a man. This moment is the burden I carry."

Jeff awakes in the middle of the night. Sometimes, he wanders the house. He'll use Google Earth to zoom in on the exact spot where Andrew died. Lori often cries herself to sleep. She wonders if she'll ever find the happiness she once had.

They try to maintain focus. "Andrew had courage. He had guts," Lori says. "So I can have guts, too. And maybe it gives his death some meaning or a purpose, that he didn't die for nothing."

The Wilfahrts speak to veterans groups, gay groups, book clubs. Their message: Our son was an American hero, not someone to be feared because he was gay.

In an Army cherry chest in the family library are Andrew's six medals, including the Bronze Star and Purple Heart. They share space with a class assignment from when he was 10.

"These people are important to me: every good person, friend, etc.," the boy wrote. "The one thing I am most thankful for is my family."

A loveseat across the room is overrun with compact discs, journals and Moleskine Music Notebooks he carried with him in Afghanistan. Inside are his scribbled music compositions.

In one leather-bound book, Andrew jotted down favorite quotes.

"Speak your mind, even if your voice shakes."

"The cave you fear to enter holds the treasure you seek."

"Too often we don't hear the deaf and do not see the blind."

"What we dream we become."

His friend, Gill, says the man he will always remember is a great American hero. "Andrew became that person he always wanted to be."

Jeff Wilfahrt always reads a poem by William Wordsworth when he visits his son's grave.

He was just two days from leave when he was killed. "With luck, I'll be home as soon as the 6th," he said in the last sentence he ever wrote his father.

Instead of greeting their son with hugs on March 6, mom and dad buried their boy. His final resting spot is among thousands of others at Fort Snelling National Cemetery, a place where Jeff and Lori now come for solitude.

A lover of literature, Jeff always brings a collection of William Wordsworth. He flips the pages to "Expostulation and Reply." He sits on the marble stone commemorating his son and reads aloud. Lori sits on the ground nearby.

He gets to the last verse and chokes up:

"Then ask not wherefore, here, alone, Conversing as I may, I sit upon this old grey stone, And dream my time away."

Jeff stands quickly, touching the grey stone with his hand, as if reaching out to his beloved son from beyond the grave. He trembles and cries. "I can never get through the last paragraph," he says. "What the hell's wrong with me?"

Lori stands, too. The two stare at the headstone. Tears still streaming down his face, Jeff says, "It's just the shits." He whispers again, "It's just the shits."

They want people to know their son wasn't a "gay soldier." He was a great soldier who happened to be gay. Above all, he was a citizen.